The sulfate or kraft process is widely used in the pulp and paper industry to convert wood chips into partially delignified cellulose pulp which is used directly in unbleached board and other unbleached paper products, or which is further delignified and bleached for making high brightness paper products. In this well-known process, the chips are converted into pulp at elevated temperatures by chemical delignification using an aqueous solution known as white liquor which contains sodium hydroxide, sodium sulfide, and other dissolved salts. The spent liquor from this process step, known as weak black liquor, contains residual organics, dissolved lignin, and other wood constituents. This weak black liquor is concentrated by evaporation, at which point soaps, resin salts, and fatty acids are recovered. The resulting strong black liquor is further evaporated, sodium and sulfur in various chemical forms are added as needed to replace sulfur losses in the process, ant the mixture is combusted in a recovery furnace to yield molten sodium sulfide and sodium carbonate; this molten material is then dissolved in water to give an aqueous solution known as green liquor. The green liquor is causticized with calcium oxide (lime) to convert the sodium carbonate to sodium hydroxide (caustic), which yields white liquor for use in another pulping cycle.
White liquor is a potential source of alkali for certain process steps in a kraft pulp mill except for the presence of sodium sulfide in the white liquor, which is undesirable in most applications. It has become common practice in kraft mills to oxidize white liquor with air to remove most of the sodium sulfide by conversion to partially oxidized sulfur compounds comprising mostly sodium thiosulfate. This yields an aqueous alkali, commonly known as oxidized white liquor, which contains sodium hydroxide and sodium thiosulfate as the major constituents with lesser amounts of sodium carbonate, sodium sulfite, and sodium sulfate, and which contains low levels of undesirable sodium sulfide. Oxidized white liquor as defined above is widely used as an alkali source in oxygen delignification, a process step which removes additional lignin from kraft pulp to produce a higher brightness pulp. The use of oxidized white liquor helps to maintain the balance of sodium and sulfur in the pulp mill because the residual alkali from oxygen delignification is returned to the white liquor cycle. Oxidized white liquor as defined above also can be used in gas scrubbing applications, for removal of residual chlorine or chlorine dioxide from bleach plant effluents, in the regeneration of ion exchange columns, and for the neutralization of various acidic streams in the pulp mill. Oxidized white liquor as described above generally cannot be used in bleaching stages which utilize peroxide, hypochlorite, or chlorine dioxide because the partially oxidized sulfur compounds consume additional bleaching chemicals in a given stage or in subsequent stages, thus rendering the use of oxidized white liquor uneconomicai in such applications. Oxidized white liquor as defined above also cannot be used as an alkali source for the production of sodium hypochlorite from chlorine and sodium hydroxide, since thiosulfate reacts with chlorine and sodium hypochlorite.
In current kraft pulp mill operation, the term white liquor oxidation means the oxidation of white liquor using air or oxygen to destroy sodium sulfide by converting most of the sulfide to sodium thiosulfate. U.S. Pat. No. 4,053,352 discloses a method of oxidizing white liquor with an oxygen-containing gas, preferably air, to convert practically all sulfides to thiosulfate. Oxidation is carried out by injecting air into white liquor in a tank at a flow rate of 50 to 500 Nm.sup.3 /(hr-m.sup.2) whereby the air provides oxygen and agitates the liquid to promote mixing. Oxidation is carried out between about 50.degree. C. and 130.degree. C. at a pressure up to 5 bars above atmospheric pressure. The use of oxidized white liquor as a source of alkali is disclosed, including applications in the steps of oxygen bleaching, flue gas scrubbing, chlorine bleaching, treating of waste gases from bleaching processes to destroy chlorine or chlorine dioxide, regenerating ion exchange columns, and neutralizing acidic liquids. Several process steps are defined for which oxidized white liquor cannot be used as an alkali source, such as peroxide bleaching and in the manufacture of hypochlorite.
In an article entitled "Use of White and Green Liquors as Alkalis in the Oxygen Stage of Kraft Pulp. (1) Oxidation of White and Green Liquors" published in Przeglad Papier 35, No. 6, June 1979, pp. 193-195, K. Baczynska reports results of a study on the oxidation of these liquors. The study found that the main oxidation product of sulfide contained in these liquors is thiosulfate; depending on the conditions of reaction, nearly complete oxidation (99.8%) of sulfide is possible but requires up to 5 hours of reaction time. In the presence of pulp in an oxygen bleaching reactor, sulfide oxidizes essentially to sulfate and very small amounts of sulfite and thiosulfate. The article teaches that white liquor oxidation to predominantly thiosulfate can be accomplished batchwise in a glass column at temperatures between 40.degree. C. and 80.degree. C. using a contacting time of 1.5 to 8 hours.
Soviet Union Patent SU 1146345 A discloses the oxidation of white liquor with a gas containing oxygen with addition or spent alkali from an oxygen bleaching stage to increase the rate of oxidation. Complete oxidation of sulfide occurs in 40 minutes at 90.degree. C. under an oxygen pressure of 0.2 MPa compared with 60 minutes when no oxygen bleaching spent alkali is added. The products formed by the oxidation of sulfide are not described.
A. I. Novikova et al in an article entitled "Oxidation of White Liquor by Oxygen" in Khim. Tekhno Ee Prorzdnykh 1985, pp. 49-52, describe the reaction paths of sulfide oxidation in white liquor using oxygen or air. It is postulated that the sulfide first oxidizes rapidly to polysulfide (Na.sub.2 S.sub.x) , sulfite, and thiosulfate. Subsequent oxidation of intermediate species to sulfate occurs slowly and catalysts are required to accelerate the reaction. Partially oxidized white liquor containing polysulfides is said to accelerate delignification when used as an alkali for delignification and bleaching; for this reason oxidation to sulfate is stated to be undesirable. Specific operating conditions for white liquor oxidation are not disclosed.
The use of pure oxygen instead of air for white liquor oxidation is described in a brochure entitled "AIRCO Tech Topics" by Airco Gases, March 1990. A pressurized pipeline reactor with recycle is disclosed for the oxidation of sodium sulfide in white liquor to sodium thiosulfate and sodium hydroxide. It is stated that the oxidation chemistry is the same whether using air or pure oxygen and that both produce a sodium thiosulfate product.
The background art summarized above thus discloses the oxidation of white liquor to destroy sulfide by conversion to a partially oxidized intermediate product comprising mostly thiosulfate. In addition, uses of such an oxidized white liquor as an alkali source in certain process steps in a kraft pulp mill are described. However, other applications are listed in the background art in which such an oxidized white liquor cannot be used as an alkali source, chiefly because it contains thiosulfate which consumes the oxidizing compounds used for bleaching kraft pulp. Specific methods to produce and use an oxidized white liquor which is free of significant amounts of thiosulfate or other partially oxidized sulfur compounds are not known or described in the current background art.
The invention disclosed in the following specification and defined in the appended claims offers methods for the selective oxidation of white liquor and the use of different selectively-oxidized white liquor products for improved kraft mill operation.